A frequency modulated signal consists of a predetermined sequence of data bits (Dbits), where each Dbit type is transmitted as a signal having one or more frequencies from a predetermined set of specified frequencies. Use of a frequency modulated signal to transmit information over a communication line requires a detector capable of quickly detecting the signal even in a low signal to noise ratio environment.
For example, communication devices are known capable of transmitting and receiving voice as well as video signals. When a voice communication link has been established between two or more patties, and one of the parties decides to change the mode of communication from voice to video, the party transmits a CI (call indication) signal to the other party's device in order to inform the other party's device to change its mode of communication from voice to video. This CI signal is specified in the 1TU-V.8 standard, and is a frequency shift modulated signal known as the “T1-N0” sequence. This sequence consists of 10 ones (‘1111111111’) followed by 10 synchronization bits (‘0000000001’) and a call function octet (‘0 byte data 1’). The standard further specifies that a “0” be transmitted as a 1180 Hz signal, and that a “1” be transmitted as a 980 Hz signal. These two frequencies, 980 Hz and 1180 Hz are referred to as the “FSK” (frequency shift key modulation) frequencies. The CI signal is transmitted with a regular ON/OFF cadence in which the ON periods are not less than 3 periods of the CI sequence, and are not greater than 2 sec in duration. The OFF periods are not less than 0.4 sec and not greater than 2 sec in duration.
A communication device capable of alternating between voice and video modes needs to include a CI signal detector, also known as a CI detector. When the CI detector identifies a CI signal received at the communication device from a remote communication device indicating that the remote device has changed from voice to video communication, the CI detector generates a signal causing its communication device to change from voice to video communication.
As another example, dual frequency (DF) signals, also known as dual-tone multiple frequencies (DTMF), are signals that are an additive combination of two equal-amplitude frequency components. Thus, the signal generated by depressing “1” on the telephone keypad is the sum of a 697 Hz and a 1209 Hz frequency signals, and the signal generated by depressing “5” is the sum of a 770 Hz and a 1336 Hz sine wave. DF signals are used for example, for representing telephone numbers and other signaling functions within a telephone system including interactive voice response.